1. Field of the Invention
The invention is directed to a pressure pulse generator of the type having a pressure pulse source for generating acoustic pressure pulses in an acoustic propagation medium by rapid displacement of a membrane from an initial position by an electromagnetic, pneumatic or mechanical drive.
2. Description of the Prior Art
Pressure pulse generators of the type described above can be employed for a large variety of purposes, for example in medicine to non-invasively disintegrate calculi situated in the body of a patient, or to non-invasively treat pathological tissue changes. Positive (greater than atmospheric) pressure pulses are employed in the former instance and preferably negative (less than atmospheric) pressure pulses are employed in the latter instance. Further, such pressure pulse generators can be employed, for example, in materials testing to charge material specimens with pressure pulses. The pressure pulse generator is always acoustically coupled to the subject to be acoustically-irradiated in a suitable way, so that the pressure pulses generated in the acoustic propagation medium can be introduced into the subject. The pressure pulse generator and the subject to be acoustically-irradiated must be aligned relative to one another such that the region of the subject to be acoustically-irradiated is situated in the propagation path of the pressure pulses. When the pressure pulse generator provides focused pressure pulses as an output, it must also be assured that the region of the subject to be acoustically irradiated is situated in the focus region of the pressure pulses.
U.S. Pat. No. 4,674,505 discloses a pressure pulse generator of the type initially described. The generator described therein is an electromagnetic shock wave generator that generates positive pulses. In this type of generator, an electrical coil arrangement builds up a magnetic field extremely quickly when charged with a high-voltage pulse. This magnetic field induces a current in an electrically conductive membrane lying opposite the coil arrangement. This current is opposite the current flowing through the coil arrangement is being thus accompanied by a magnetic field opposite the magnetic field associated with the coil arrangement. As a consequence of the resulting repulsion forces, the membrane is suddenly moved away from the coil arrangement. A pressure pulse is thereby introduced into an acoustic propagation medium adjacent the membrane, which gradually intensifies into a shock wave along its propagation path.
A problem in pressure pulse generators of this type is that the membrane must be returned into its initial position after generating a pressure pulse. Only then it is guaranteed that the membrane assumes a defined initial position before generating the next pressure pulse; the membrane lying flush against the coil arrangement in this initial position in the case of electromagnetic pressure pulse generators. Return to a defined initial position is a prerequisite for successively generated shock waves coinciding with adequate precision with respect to their acoustic characteristics. In a pressure pulse generator disclosed in European Application 0 188 750, corresponding to U.S. Pat. No. 4,697,588, the return of the membrane to its initial position is accomplished by charging that side of the membrane facing away from the acoustic propagation medium with negative pressure. Although a reliable return of the membrane to its initial position is guaranteed in this way, a substantial structural outlay must be provided, as must a negative pressure source.
German Published Application 34 43 295 also discloses a pressure pulse generator of the type described above whose membrane is returned into its initial position by the acoustic propagation medium that resides at a static pressure. This technique has the disadvantage that the acoustic propagation medium adjoining the membrane cannot be conducted through a cooler in a circulation loop during operation in the way known from European Application 0 265 741, corresponding to U.S. Pat. No. 4,977,888, without additional outlay. Such cooling is necessary for eliminating the dissipated heat that arises during operation of the pressure pulse source to prevent premature failure of the pressure pulse source because of inadmissibly elevated operating temperatures, particularly due to failure of the membrane subject to high mechanical stresses.